The trouble is I'm unsure how to get a clean signal. When I feed one (photoreflective) sensor output into the Arduino (A0 input), the signal is clean. When I feed it into one of the Ethernet lines (say orange), and take the output at the other end to feed it into the Arduino, the signal is all over the place.

Is this noise caused by the cable picking up my PC's electromagnetic interference? Or is the signal being encoded somehow? What is the proper way to hook up the sensor output to the network cable? For example, do I have to have one line for signal and another (matching color) for ground? Is the cable powered through the sensor output, or should I be powering it at the Arduino end?

A friend suggested I stick with regular shielded coaxial cable, one per sensor output. But I was hoping to get 4 sensor inputs (1 per cable pair) running through a single network cable to simplify.

Anyway, any clarification would be a huge help at this point. I'm quite new to this. Thank you for your patience!

Here are the sensors I'm using (8 in all):http://www.sparkfun.com/products/246

The cables will extend around 10 feet.

UTP looks to be pretty much the same as Ethernet/Cat6; I'm not sure, but the Cat6 cable I have may be unshielded too, which might be the reason I'm getting interference (being next to a PC).

I don't have a schematic; the QRD1114 circuits work though, and I'm temporarily testing by sending the sensor outputs to the Arduino via audio cable, which also works.

Still, is it a simple matter of connecting a sensor output to one of the Cat6 inputs, then taking that at the other end of the Cat6 and feeding it into the Arduino analog in? Because right now that's not working.

At the moment yes: since I'm working off the breadboard, all sensors are going to the GND pin on the Arduino. But the installation will ultimately be designed so that each sensor operates off its own battery power (each one will be in a separate location in the hall), so each one will have its own ground.

I believe you still need to wire all grounds together. So one of the wires in your cat6 needs to be the common ground. Also, I believe you said that you were using unshielded cable. You probably should get proper shielded cables as well.

So let me see if I've got this. Taking three arbitrary wires on the Cat6, wire 1 supplies power to sensor from the Arduino, wire 2 transmits sensor data back to Arduino, and wire 3 acts as common ground. Do I have that right?

And yes, making sure the cable is shielded will help. Thanks for the feedback.

According, all information you write it spouse to be problem with common ground. Please check wiring diagram for 1-Wire system by Hobby Boards https://www.hobby-boards.com/store/pages.php?pageid=11 , and try to reimplement for your purposes.

I've been looking into alternatives to Ethernet cables, specifically using RS-485 / MAX 485 to convert incoming analog sensor data from the 8 sources I mentioned. From what I've read, this will greatly increase the distance over which I can transmit.

But in order to cut down on cable clutter, I'm still wondering what the best cable option would be. Obviously something shielded with low impedance. Some kind of twisted pair? Shielded audio cable? Coaxial with BNC connectors? What gauge? Which then raises the question of which perfboard terminals and connectors I'd need. The choices are a little bewildering at this stage, so if anyone has done this before, I am all ears.

Again, I am estimating the cable distance, one-way, to be around 20-25 feet. And I'm guessing I'd need something like 2 cables with at least 4 lines each, not counting power and ground conductors.